1. Field of Endeavor
The present invention relates to gas turbine engines, and more specifically to a method for protecting a gas turbine engine, including a compressor, a combustor and a turbine, against high dynamical process values.
It also relates to a gas turbine engine useful for conducting such a method.
2. Brief Description of the Related Art
Pulsation measurement equipment is an integral part of a gas turbine protection system. The purpose of the equipment is to supervise combustion phenomena like pulsation (screeching, rumbling, humming, etc.) to cause an immediate engine shut down or to initiate a change in the engine operation point in the event of sudden high pulsations or long lasting moderate pulsation levels in order to protect the engine from severe damage.
Pulsation monitoring on gas turbines is required because the combustion is operated close to the lean extinction limit to reduce emissions. The combustion flame instabilities are interacting with the flow and generate powerful acoustic waves. These waves excite the thermally highly loaded structures of the combustor. If the mechanical vibrations of combustor hot gas parts match in some ways with the acoustic pulsation load, significant wear, leading to a lifetime reduction or even immediate failure, occurs.
The pulsations in a gas turbine combustor are usually measured directly inside the combustor in order to have an accurate measurement across the full frequency spectrum of the pulsations. The conversion of the physical hot gas pressure fluctuation into an electric output signal (high frequency, voltage or current or digital) is performed by a high temperature dynamic sensor (microphone, dynamic pressure transducer, dynamic pressure pick-up, piezo-electric, piezo-resistive, capacitor, strain gage, optical or any other principle), which is combined with suitable signal conditioning (the specific working principle of the sensor is not relevant to the invention disclosed below).
However, the direct strain or acceleration measurement on a single component is not considered here. This kind of approach is limited to the structure being measured and the signal cannot be used to infer the stresses in other components not measured. The acoustic signal represents the direct excitation function to the complete structure.
The conditioned sensor signal is subsequently processed, cut into one or more supervision frequency bands, which are individually analyzed and evaluated by a suitable algorithm in the gas turbine protection logic. The filtering of the signal is usually done by analog filters. Their filtering characteristics allow for a certain crosstalk between neighboring bands. A better filtering procedure is described in document EP 1 688 671 A1 or U.S. Pat. No. 7,751,943, where the band pass information is extracted in the frequency domain. The amplitudes of these filtered bands are then used as a measure to change the engine operation point (e.g., de-rating the power output) or to shut down the engine immediately or use it as a feedback control parameter in active combustor controls (ACC) systems for performance optimization and instability control.
The pulsation of an engine is not fully predictable and depends on various parameters: fuel quality, fuel/air ratio, combustion operation modes, fuel systems failures, wear of seals, compressor mass flow changes, hydrodynamic time lags, etc. The weakness of the currently used engine protection method is that the parameters for the decision thresholds (pulsation severity, time delay, frequency bands) for initiating a certain engine protective action (alarm, operation point adjustment, trip, etc.) is up to now always based on weak empirical evidence using damage profiles of inspected hardware. This limits the confidence to older engine designs, which have experienced several inspection cycles and prevent the definition of any protection for new Gas Turbine Systems for which no field experience is available.
The combustion settings of newly installed or serviced engines are usually optimized for power, efficiency and lifetime using the spectral information of the dynamic pulsation signal. During this procedure, the combustion parameters are usually tuned in such a way as to reduce the highest peak of the dominant acoustic mode. This may be misleading for the following reasons:
a) it is currently not possible to define a maximal allowed pulsation level of a certain peak or frequency band for continuous engine operation using only wear marks of hot gas components. The empirical approach provides no answers between the coupling of the acoustics excitation and the structural failure modes;
b) the highest peak may not be the only one responsible for the structural resonances, while some lower pulsation peaks might be exciting the structure directly.
These engine adjustments are also needed when engine components receive an upgrade (e.g., compressor blading upgrade). The pulsation spectrum may then be different and the empirical basis for an optimal engine adjustment and protection needs to be re-established, which may take several years of feedback.
The current state of the art in pulsation monitoring of heavy-duty gas turbines is well described by the reference “Technical Progress Report; Castaldini, C., CMC-Engineering; Guidelines for Combustor Dynamic Pressure Monitoring, EPRI, Palo Alto, Calif., 2004, Product ID: 1005036”. Pulsation monitoring is now a standard tool to adjust a heavy-duty gas turbine, to protect the engine from immediate dangerous pulsation peaks (see for example documents EP 1 688 671 A1 or U.S. Pat. No. 5,719,791), as well as use for automated feedback (see document GB 2 042 221 A) for emission control and active damping or just to monitor the combustor acoustics by recording the trends in order to meet the combustor lifetime specifications. The main focus in the development of pulsation monitoring in the past was mainly directed towards the sensor side to reach higher operating temperatures (see, for example, product leaflet “New dynamic pressure sensors” of MEGGITT, 2010) and signal interfaces (EP 1 688 671 A1).
While there are some documents that describe active control of pulsations by use of a feedback loop (e.g. EP 1 327 824 A1) there are only few patents that describe the use of the signal for engine protection actions (EP 1 688 671 A1). The protection settings are usually defined by using empirical evidence of known damages (see Technical Progress Report; Castaldini, C., CMC-Engineering; Guidelines for Combustor Dynamic Pressure Monitoring, EPRI, Palo Alto, Calif., 2004, Product ID: 1005036).
There is one document, however, which describes a method to estimate the lifetime of the combustor based on the acoustic signal (EP 1 995 519 A2). The method according to that document uses the frequency and amplitude of the acoustic oscillations as a direct cyclic load input to assess the cumulative damage of the material used in the combustor (fatigue). A cumulative damage value is computed which, when reaching a specific value, is used to initiate a command to switch off the turbine to initiate an inspection. This approach assumes that the acoustic modes are exciting the combustor structure directly at that frequency and that the material stress is directly a function of the amplitude of the acoustic frequency, which is misleading.
The disadvantages of that known method are as follows:                This approach does not take structural resonances into account. It takes every frequency as an equal contributor to the cumulative damage of the material according to its amplitude. Structural resonances can amplify the stresses in a structure many times over a normal vibration level. Some frequencies may not be dangerous at all since no structural resonance is excited.        The method does not provide any information about the best adjustment of the engine parameters for optimal lifetime. It is passive and just checks the accumulated cycles. Due to the fact that many engine parameters have an effect on the combustor pulsations, the operation point of a gas turbine can be set to provide the best lifetime of the hot gas parts.        The method can lead to early and unnecessary engine shutdowns because the accumulated damage coefficient is based on all frequencies of the pulsations. The engine availability can therefore be affected.        
The state of the art in monitoring and controlling combustor pulsations is as follows:
Pulsation signals are recorded with a suitable high temperature sensor at the combustor and analyzed by analog or digital (FFT) band pass filtering (e.g., U.S. Pat. No. 7,751,493 B2).
Representative frequency bands are extracted or specific frequency peaks are tracked. The rms values of the bands (U.S. Pat. No. 7,751,493 B2) or the amplitudes of the principal peaks are used as a measure for the severity of the pulsations.
The rms value or peak amplitude is compared against a threshold value which, when exceeded, initiates a protective measure on the gas turbine after a specific delay time (U.S. Pat. No. 7,751,493 B2) or provides a control signal to move the operation point of the engine (e.g., load change). Simpler concepts just consider regular checks of the frequency spectrum for shifts of the dominant peaks.
The threshold value is usually defined based on engine failure diagnostics feedback (wear analysis, damage pattern analysis, see Technical Progress Report; Castaldini, C., CMC-Engineering; Guidelines for Combustor Dynamic Pressure Monitoring, EPRI, Palo Alto, Calif., 2004, Product ID: 1005036). There is no specific description in the literature or in patent documents of a direct method that defines the correlation between acoustic amplitudes and frequencies with regard to hot gas parts failures. The only reference found, described in document EP 1 995 519 A2, tries in a simplified way to predict the lifetime of the combustor material. It fails to provide a link between the driving acoustic modes and the resulting hardware vibrational modes and material stresses.
Engine adjustments are done based on pulsation frequency spectra, where the largest peak is usually reduced by adjusting the principal combustion parameters. With the available knowledge, there is no possibility to have a definite rule which pulsation peaks affect most the component lifetime.
Structural models for mechanical integrity and lifetime assessments of structures or even complete gas turbines are state of the art and part of the engine development and design process.
There is currently no method available that links the measured acoustic pulsation data with the achievable lifetime of the combustor structure. Engine adjustments are performed with the target to reduce the principal pulsation peaks, which is not the optimal approach. The conservative settings do not allow utilizing the full engine potential (power, efficiency) and may be even misleading in the lifetime prediction, since not the most damaging frequencies are reduced.